1. Field of the Invention
The present invention relates to the field of wireless communications, more particularly to synchronizer performance within a wireless modem.
2. Description of the Related Art
Explosive growth in the market for internet and intranet related applications has provided the impetus for a greater demand for fixed wireless networking services and systems. A wireless internet access system (WIAS) illustrated in FIG. 1 is composed; of four major parts: (a) multiple data base stations (BS) 100(a) and 100(b) which provide wireless connectivity and gain coverage to subscriber units 102(a)-(d) of a large geographical area (for example, residential and corporate terminal equipment as illustrated in FIG. 1); (b) wireless modems 170(a)-(c) (hereinafter xe2x80x9cWMxe2x80x9d) which are connected to BS 100(a) or 100(b) via wireless links 115(a)-(c); (c) a data switching center (DSC) 125 with integrated management functions; and (d) a backbone transmission network 135 interconnecting (a)-(c) above.
As can be seen from FIG. 1, corporate terminals 102(c) and 102(d) can be, and many times are, connected to WM 170(c) via a local area network (LAN) and a wireless router or firewall (not shown). Additionally, BS 100(a) and 100(b) may communicate with DSC 125 via frame relays (not shown). Further in conventional wireless internet access systems or networks, DSC 125 is interconnected with backbone transmission network 135 by a router and/or firewall (not shown for clarity).
FIG. 2 illustrates BS 100(a) and 100(b) of FIG. 1 in an operational mode. Each BS 100(a) and 100(b) provides 360xc2x0 RF coverage on the order of several gigahertz (preferably operating in the 3.5 GHz spectrum using approximately 5 MHz wide channels), sending and receiving signals over air lines 115(a)-(c) between individual subscriber units 102(a)-(d) served by BS 100(a) and/or 102(b). More particularly, the designated geographical area of subscribers served by each BS 100(a) and 100(b) is typically called a cell 150, defined by its coverage area as shown in FIG. 2, where BS 100(a) and 100(b) are situated in designated cells 150(a) and 150(b). Within each cell 150(a) or 150(b) reside a plurality of subscribers 102(a)-(d) served by the BS 100(a) and/or 100(b) includes a plurality of access points (hereinafter xe2x80x9cAPxe2x80x9d, not shown in FIG. 1) serving as an interface between individual subscribers 102(a)-(d) of a cell 150(a)-(b) served by BS 100(a)-(b). Each access point includes receiver and transmitter circuitry of the base station for communicating with individual subscribers 102(a)-(d) within a designated cell 150(a)-(b).
Due to the need for increasing frequency spectrum reuse in the gigahertz band, in an effort to conserve this precious resource, the trend has been to reduce cell size even further (to microcells or picocells) which cover an even smaller geographical area, or which can serve hard to reach areas such as gullies and depressions where subscribers reside. Unfortunately this beneficial effect of increasing frequency spectrum reuse is offset by an increasing chance of neighboring cells interfering with each other, causing loss or degradation of the wireless signal. This loss or degradation of the wireless signal may be caused by, for example: (a) Rayleigh fading or delay spread due to multipath propagation; (b) shadow fading due to obstructions from natural and man-made objects around the main transmission path of the subscriber""s devices; and (c) interference between co-channels and/or adjacent channels of wireless networks serving the subscriber""s devices.
One particular problem related to (a) above could result from the development of signal delay spread in the wireless channel between a WM and an AP. A channel is the wireless link between a WM antenna and an AP antenna. A WM can function in at least five different frequency bands, but it only works in one frequency band, or one channel, at a time for receiving packets of information transmitted by an access point (AP), for example. Within the receiver circuitry of a WM is a synchronizer which perform an algorithm for time and frequency synchronization between received packet information and the receiver. The AP typically uses the same synchronizer algorithm as the WP. The synchronizer determines the starting time of an incoming packet and estimates the frequency offset between the transmitter of the AP and the receiver of the WM, so as to process the detected packet information.
A channel can go bad due to a variety of environmental conditions or changes, such as that due to traffic, temperature, rain, foliage, etc. For example, the terrain of a geographical area served by a wireless network can create multi-path delay spread of radio propagation. Multi-path delay spread in turn creates inter-symbol interference in the receiver detection circuitry, which ordinarily should be remedied by an equalizer component within the receiver. The equalizer, as well as the various components of a receiver are discussed further below.
For mobile systems, severe delay spread channels may be avoided by moving the mobile systems from place to place. However fixed wireless systems, employing a Time Division Multiple Access (TDMA) air interface for example, do not have the flexibility to be moved around in order to reduce the effects of severe delay spread channels. Thus the receivers within these fixed WMs need to be as robust as possible in order to handle delay spread channels and the effects thereof, which are discussed below.
A severe delay spread channel can usually be determined by examining the impulse response of the channel, or its h(z) function. If the frequency response of h(z) is relatively flat, the channel is a good channel in the sense that the inter-symbol interference is not so severe, or may be adequately handled by the equalizer within the receiver. However, if the frequency response exhibits a deep null, this is indicative of a bad channel, and the inter-symbol interference resulting from this spread will be difficult to equalize. For example, a bad channel could develop if the receiver is receiving from more than one strong signal source, and these two signal sources are separated by some time delay longer than xe2x80x9cone symbolxe2x80x9d time due to the multi-path effect described above.
To understand how the current synchronizer works, and also to comprehend the effects of delay spread on modem performance, the following terms are defined. Each detected packet is divided into segments allocated to various components within the receiver. The synchronizer segment of an incoming packet contains 17 symbols. There are eight time samples per symbol allocated in the synchronizer segment. A bin is a storage location for storing a corresponding one of the eight samples for each sequentially processed symbol; thus there are eight bins in the synchronizer, bins b1 to b8.
The synchronizer first wants to determine if there is any differential phase error (DFE) test failure in each bin. Because these 17 transmitted sync symbols are always known to the receiver beforehand, the receiver compares the phases of the received samples in a certain bin with the phases of those known sync symbols. Suppose that the phases of sync symbols are ∠x(1), ∠x(2), . . . , ∠x(17). The receiver checks if bin 1 has a DFE test failure by first looking at the difference between |∠x(1)xe2x88x92∠x(2)| and |∠s1xe2x88x92∠s9|. This difference is called DFE. If the absolute value of DFE is larger than 90xc2x0, a DFE test failure occurs. Next the receiver checks the difference between |∠x(2)xe2x88x92∠x(3)| and |∠s9xe2x88x92∠s17|, and so on. If all 16 DFEs are  less than 90xc2x0, then bin 1 does not have a DFE test failure.
In the current algorithm, the synchronizer chooses a bin location only if the bin does not contain any failure of the differential phase error test. If all bins have phase test failures, the synchronizer fails and the received packet is xe2x80x9cthrown awayxe2x80x9d. This means that it is as if the receiver never received the transmitted packet; since the synchronizer has failed, none of the follow on processes in the receiver are performed and the packet must be re-transmitted.
Under severe delay spread channels, there is a good likeliness that all bins will have DFE test failure(s). Therefore, a problem with the current method of synchronizer operation is that it is too sensitive in regard to possible severe delay spread channels. What is desired is an algorithm that allows continued performance of the synchronizer despite the presence of delay spread channels, in order to maintain communication connectivity with an access point (AP) in a wireless network serving subscribers, for example.
FIGS. 3a and 3b illustrate a signal amplitude for a channel which is not subject to delay spread and a channel which is subject to the influence of delay spread. Referring to FIG. 3a, there is illustrated a signal amplitude of I/Q real and imaginary parts of the signal), a channel that does not introduce any delay spread. The circles represent the I signals and the squares represent the Q signals. These signals represent the actual seventeen synchronization symbols processed in a synchronizer of a WM or an AP, where I and Q are always the same value (both either +1 or xe2x88x921).
However, as illustrated in FIG. 3b, the same set of symbols are not matched under the influence of a delay spread model. The channel response for FIG. 3(b) is defined as:
h(z)=0.707+0.707*ejxcfx80/2zxe2x88x921
In this delay spread model, zxe2x88x921 is indicative of a one symbol time delay, and ejxcfx80/2 represents that a transmitted symbol is rotated clockwise by 90 degrees on the complex plane. For example, and given the impulse response, if an AP transmitter transmits signals s1, s2, s3, etc., the WM receiver sees 0.707(s2+is1), 0.707(s3+is2), etc.
Thus in FIG. 3(b), the received I and Q signals are completely out of phase. This out of phase characteristic is obviously disadvantageous, given the current bright line test regarding differential phase error test failures in the synchronizer. Specifically, if each bin contains a failure (in this case each bin would have a differential phase test error failure) the synchronizer fails in toto. This in turn causes the receiver to discard the rest of the packet and the transmitter has to retransmit the same packet again. Thus, overall system data throughput suffers from this iteration. Therefore what is needed is a more robust frequency/time synchronization algorithm which is less sensitive to these delay spread effects of bad channels.
The present invention provides a method for synchronizing a transmitted packet with a receiver in a wireless communication system. The method includes assigning a plurality of samples in a received synchronization segment of an incoming packet to corresponding bins, determining a number of phase test failures in each bin and selecting a bin having the fewest number of phase test failures for synchronizing the time of the incoming packet with the receiver. Additionally, the phase drift is calculated for the selected bin so that the frequency of the incoming packet is synchronized with that of the receiver.